1. The Field of the Invention
The present invention generally relates to x-ray generating devices. More particularly, the present invention relates to an x-ray tube rotor assembly having superior cooling characteristics.
2. The Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device.
The x-ray tube generally comprises an outer housing in which is disposed a substantially cylindrical vacuum enclosure. The vacuum enclosure has disposed therein a cathode and an anode. The cathode includes a filament that, when heated via an electrical current, emits a stream of electrons. The anode typically comprises a graphite substrate upon which is disposed a heavy metallic target surface that is oriented to receive the electrons emitted by the cathode. Though some x-ray tube anodes are stationary, many are rotatably supported within the vacuum enclosure by a rotor assembly.
The rotor assembly typically comprises a rotor shaft, a rotor hub and sleeve, a bearing assembly and a magnetic sleeve. One end of the rotor shaft supports the rotary anode, while the other end is attached to the rotor hub and sleeve. The rotor hub interconnects the rotor shaft and the rotor sleeve with the bearing assembly, thereby enabling the shaft and sleeve to rotate. The rotor sleeve is rotationally and concentrically disposed about a substantial portion of the bearing assembly. A stator is used to induce rotation of the rotor sleeve, which in turn causes the rotor shaft and anode to rotate. The magnetic sleeve typically attaches to and covers either the outer surface of the bearing housing or the inner surface of the rotor sleeve to assist the stator in inducing rotation of the rotor sleeve.
In order for the x-ray tube to produce x-rays, an electric current is supplied to the cathode filament of the x-ray tube, causing it to emit a stream of electrons by thermionic emission. A high voltage potential placed between the cathode and the anode causes the electrons in the electron stream to gain kinetic energy and accelerate toward the target surface located on the anode. Upon striking the target surface, many of the electrons convert their kinetic energy into electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (xe2x80x9cZ numbersxe2x80x9d), such as tungsten carbide or TZM (an alloy of titanium, zirconium, and molybdenum) are typically employed. Finally, the x-ray beam passes through windows defined in the vacuum enclosure and outer housing, where it is directed to an x-ray subject, such as a medical patient.
A recurrent problem encountered with the operation of x-ray tubes deals with the removal of heat from tube components. In general, only a small percentage of the electrons that impact the anode target surface during x-ray production do, in fact, produce x-rays. The majority of the kinetic energy is instead released as heat that is absorbed into the anode target surface and surrounding areas. This heat must be continuously and reliably removed from the anode and surrounding components in order to prevent damage to critical tube components. To the extent that the heat is efficiently removed, less thermal and mechanical stress is imposed upon the x-ray tube, and its operation and performance will be enhanced. If the heat is allowed to reach detrimental levels, however, it can damage the anode and/or other tube components, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube.
Many approaches have been implemented to help alleviate the problems created by heating within the x-ray tube. For instance, as noted the anode in many x-ray tubes is rotatable. During operation of the x-ray tube, the rotary anode is rotated at high speeds, which causes successive portions of the target surface to continuously rotate into and out of the path of the electron beam produced by the cathode filament. In this way, the electron beam is in contact with any given point on the target surface for only short periods of time. This allows the remaining portion of the surface to cool during the time that it takes to rotate back into the path of the electron beam, thereby reducing the amount of heat that is absorbed by the anode at any given location.
While the rotating nature of the anode reduces the amount of heat present at the target surface, a large amount of heat is still absorbed by the anode substrate and other components within the vacuum enclosure. Of particular concern is the heat that is conducted from the anode to the rotor assembly, and specifically to the bearing assembly. Excessively high temperatures produced in the anode and conducted through the rotor shaft to the bearing sets can melt the thin metal lubricant that surrounds the bearings. This can cause the lubricant to disperse and expose the bearings to excessive friction. The lubricant may also form clumps in the presence of excessive heat, which in turn causes the bearing assembly to create excessive noise and mechanical vibration during tube operation. Such conditions can reduce the x-ray tube""s operating efficiency and even image quality. Repeated exposure to high temperatures can gradually degrade the integrity of the bearing surfaces and reduce their useful life or even cause premature bearing failure. Therefore, it is important to reliably and continuously dissipate heat from the x-ray tube, and particularly from the bearing assembly.
In an effort to remove large quantities of heat within the x-ray tube, rotor sleeves have been designed to absorb heat from the rotor shaft and then to radiate that heat to the surrounding vacuum enclosure. While assisting in limiting the amount of heat transmitted by the rotor shaft to the bearing assembly, this approach alone may not be sufficient to prevent large quantities of heat from reaching the bearing sets.
Another technique used for removing heat from an x-ray tube is to place the vacuum enclosure within an outer housing, as mentioned above. The outer housing serves as a container for a coolant, such as a dielectric oil, which surrounds and envelops the vacuum enclosure, and which may be continuously circulated by a pump about the outer surface thereof. As heat is emitted from the x-ray tube components (the anode, support shaft, etc.), it is radiated to the outer surface of the vacuum enclosure, and then at least partially absorbed by the dielectric oil. The heated oil is then passed to some form of heat exchange device, such as a radiative surface, to be cooled. The oil is then re-circulated by the pump back through the outer housing and the process repeated.
While assisting greatly in the dissipation of heat from the x-ray tube, the coolant is a only of partial assistance when attempting to directly remove heat from the bearing housing. This is due to the fact that in typical x-ray tubes, the coolant is only able to directly circulate past a small portion of the bearing housing, namely the bearing shank, which is disposed at the bottom of the bearing assembly. The rest of the bearing housing is typically prevented from direct contact with the coolant by the surrounding vacuum enclosure. Because of this typical design, effective cooling of the bearing assembly, and specifically the bearing sets, is difficult to achieve.
In light of the above discussion, a need exists to provide adequate cooling to the rotor assembly of an x-ray tube, and particularly to the bearing assembly, thereby avoiding the problems outlined above.
The present invention has been developed in response to the above and other needs in the art. Briefly summarized, embodiments of the present invention are directed to an x-ray tube rotor assembly having a structure that enables sufficient cooling thereof during tube operation. In particular, an x-ray tube utilizing the rotor assembly as disclosed and described herein is better able to reduce excessive heating of the bearing sets that may undesirably occur with known tube designs.
In a first embodiment, the present x-ray tube rotor assembly generally comprises a shaft assembly, a bearing assembly and a magnetic sleeve. Both assemblies and the magnetic sleeve are either disposed substantially within, or are attached to a vacuum enclosure, which in turn is preferably disposed within a coolant-filled outer housing. The coolant, such as a dielectric oil, is first circulated through the outer housing to remove heat from the x-ray tube, then through a heat exchanger to cool it before being re-circulated into the outer housing. The present rotor assembly cooperates with the coolant to achieve effective and continuous cooling of the assembly.
The shaft assembly of the present rotor assembly comprises a rotor hub from which extends a rotor shaft that supports the anode. Extending from rotor hub in the opposite direction is a hollow, cylindrical rotor sleeve that concentrically envelops a substantial portion of the bearing assembly. The shaft assembly of the rotor assembly is cooperatively attached to the bearing assembly via a bearing shaft, which enables rotation of the shaft assembly.
The bearing assembly of the present rotor assembly generally comprises the bearing shaft, bearing sets, a bearing housing and a magnetic sleeve. The bearing housing includes an axial cavity in which is disposed the bearing shaft. Two bearing sets are interposed near either end of the axial cavity between the bearing housing and the bearing shaft, to enable rotation of the bearing shaft relative the bearing housing. The base of the bearing housing comprises a shank that is supported by a collet.
The magnetic sleeve comprises an open, hollow cylinder having circular first and second ends. The magnetic sleeve is attached at its first end to the outer surface of the bearing housing such that it is concentrically disposed between the outer surface of the housing and the inner surface of the rotor sleeve. The second end of the magnetic sleeve is hermetically attached to the lower end of the vacuum enclosure such that the enclosure is structurally supported by the sleeve. A sealing ring is preferably interposed between the vacuum enclosure and the magnetic sleeve to enhance the seal therebetween.
The attachment of the first end of the magnetic sleeve to the outer surface of the bearing housing is such that a longitudinally extending gap is defined between the inner surface of the magnetic sleeve and the housing. The gap extends for the length of the magnetic sleeve, and is in fluid communication with the coolant disposed about the vacuum enclosure. This enables coolant to infiltrate the gap and directly circulate about a significant portion of the outer surface of the bearing housing.
During operation of the x-ray tube, heat absorbed by the rotor shaft from the anode is partially directed through the rotor hub to the rotor sleeve. This heat is partially radiated outward from the rotor sleeve toward the vacuum enclosure, but is also radiated inward toward the outer surface of the magnetic sleeve. The heat is absorbed by the outer surface of the magnetic sleeve, then transferred by the inner surface of the magnetic sleeve to the coolant circulating within the gap. Upon exiting the gap, the coolant completes its travel through the outer housing before exiting the tube for cooling prior to recirculation. In one embodiment, emissive and absorptive surfaces are preferably disposed on the rotor sleeve and magnetic sleeve to facilitate the radiation of heat therebetween. The above heat removal process occurs continuously during operation of the x-ray tube.
In addition to facilitating enhanced heat removal from the rotor sleeve and magnetic sleeve, the present rotor assembly also assists in directly cooling the bearing housing. By virtue of its proximity to the gap, a significantly larger portion of the outer surface of the bearing housing is in direct contact with circulating coolant disposed within the outer housing of the x-ray tube. Thus, augmented heat transfer between the bearing housing and the circulating coolant is achieved as compared with prior art bearing assemblies.
In an alternative embodiment, fluid passageways are defined in the collet to facilitate enhanced circulation of coolant in the gap, thereby leading to even more effective rotor assembly cooling. Further, a plurality of tubes may be disposed in the fluid passageways to direct the flow of coolant within the gap for increased heat transfer.
In another alternative embodiment, the outer periphery of the gap is not defined by the magnetic sleeve, but rather by a cylindrical sleeve extending from the bearing housing to the bottom of the vacuum enclosure. This design may be used, for instance, where the magnetic sleeve is not attached to the bearing housing as described in the first embodiment, but is rather affixed to the inner surface of the rotor assembly.
As a result of the design of the present invention, heat removal from the rotor assembly is greatly enhanced. Specifically, relatively greater heat transfer through the rotor sleeve and the bearing housing work to prevent excessive build up of heat within the bearing assembly, thereby reducing the possibility of damage to the bearing sets disposed therein. Thus, the longevity of the rotor assembly is improved and/or the ability of the tube to be run at higher anode operating temperatures is increased.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, as set forth hereinafter.